1. Field of the Invention
This invention relates to an in-cylinder pressure detection device and method for an internal combustion engine, for detecting pressure within a cylinder of the engine, and an engine control unit.
2. Description of the Related Art
Conventionally, there has been disclosed an in-cylinder pressure detection device e.g. in Japanese Laid-Open Patent Publication (Kokai) No. 2006-138205. In the in-cylinder pressure detection device, pressure within a cylinder is detected as a detected in-cylinder pressure by an in-cylinder pressure sensor. Further, by taking into considering, for example, that the output characteristics of the in-cylinder pressure sensor drift according to temperature, to compensate for the drift of the output characteristics of the sensor, the detected in-cylinder pressure is corrected using a motoring pressure. The motoring pressure, which represents pressure generated in the cylinder during a non-combustion period, is calculated by calculating a basic value thereof by the gas state equation according to the amount of intake air drawn into the cylinder, an intake air temperature, and the volume of the cylinder (combustion chamber) dependent on a crank angle, and correcting the basic value by correction parameters. The correction parameters are respectively formed by a correction coefficient by which is multiplied the basic value, and a correction term added to the resulting basic value. The correction coefficient and the correction term are both set based on the rotational speed of the engine and the intake air amount.
Then, the correction parameters for correcting the in-cylinder pressure are identified e.g. by a least-squares method, using the detected in-cylinder pressure and the motoring pressure obtained during a compression stroke such that the difference between the detected in-cylinder pressure and the motoring pressure becomes minimum. The correction parameters are also respectively formed by a correction coefficient and a correction term, and the detected in-cylinder pressure is corrected by multiplying the detected in-cylinder pressure by the correction coefficient, and further adding the correction term to the resulting detected in-cylinder pressure, whereby a final in-cylinder pressure is calculated. Further, in the in-cylinder pressure detection device, it is determined that there has occurred a misfire, when the ratio between the motoring pressure calculated during a combustion stroke and the detected in-cylinder pressure detected during the same combustion stroke and corrected as described above is smaller than a predetermined threshold value.
As described above, in the conventional in-cylinder pressure detection device, the motoring pressure is calculated by applying the correction parameters set based on the rotational speed of the engine and the intake air amount to the basic value calculated using the gas state equation, as described above. Therefore, for example, when the atmospheric pressure changes, an actual in-cylinder pressure changes according to the change in the atmospheric pressure, whereas the change in the atmospheric pressure is not reflected on the motoring pressure, which makes it impossible to properly calculate the motoring pressure. As a result, the correction parameters for correcting the in-cylinder pressure, which is identified based on the motoring pressure and the in-cylinder pressure deviate from proper values, and the final in-cylinder pressure corrected using the correction parameters also deviates from a true value, which degrades detection accuracy. Further, when determinations or detections are performed based on the relationship between the corrected in-cylinder pressure and the motoring pressure, e.g. as when a misfire is determined by the conventional in-cylinder pressure detection device, or when similar determinations or the like are performed based on the identified correction parameters, the determination or detection accuracy is also degraded.